@Nullable
    File getDestinationDir();

    /**
     * Listener triggered when a spec is added to the hierarchy.
     */
    interface CopySpecListener {
        void childSpecAdded(CopySpecAddress path, CopySpecInternal spec);
    }

    /**
     * A visitor to traverse the spec hierarchy.
     */
    interface CopySpecVisitor {

    }

    private void fireChildSpecListeners(CopySpecAddress path, CopySpecInternal spec) {
        for (CopySpecListener listener : listeners) {
            listener.childSpecAdded(path, spec);
        }
    }

    @Override
    public void visit(CopySpecAddress parentPath, CopySpecVisitor visitor) {
        visitor.visit(parentPath, this);
        int childIndex = 0;